It’s hard to see from the video, but this could be tick-order issue between the character and the camera.
If you’re already using C++, I suggest looking at the ShooterGame template on the Launcher to see how you would handle this. The gist of it is that you override UpdateCamera() in your own camera manager class, then feed the camera transform back to the players’ pawn.
The pawn will then update the transform of the first-person mesh to be relative to the camera. UpdateCamera() is called to process camera adjustments like camere shake and camera animations, so it’s safest to do it from there when you know the camera transform will be up-to-date.